1. Field of Technology
The present invention generally relates to computer networks and, more particularly, to wireless networking.
2. Description of Related Art
Throughout the 1980s, local area networks (“LANs”) emerged as a new technology market. Aided by technology that allowed LANs to operate over the structured unshielded twisted pair telephone wire already installed in most buildings, the networking market has become a $7 billion a year business. Unfortunately, as organizations grow more dependent on these high speed LANs, structured wiring systems have become less palatable. Current LANs are highly dynamic, marked by constant moves, additions and configuration changes to keep the system operating at peak performance. As a result, operating and maintenance costs are growing as users attempt to keep pace with this rapidly evolving environment. Typically, more than 60 percent of conventional LAN reconfiguration costs are attributable to labor, with the percentage much higher in metropolitan areas where labor is more expensive. For example, a conventional high speed LAN (e.g. a 30 node LAN) typically requires an average of three weeks to plan and install. In addition, more than 35 percent of problems with LANs using hubs and structured wiring and 70 percent of problems with LANs without hubs and structured wiring typically are attributable to the cabling of the LAN. In particular, the costs to move an average Ethernet unshielded twisted pair (“UTP”) connection of such a LAN average approximately $500 or more (in parts and labor) per node. This cost further increases in major metropolitan areas where increased labor costs and a prevalence of fiber-based LANs exist.
In the early 1990s, an alternative to wired LANs was the wireless LAN (“WLAN”). The benefits of this wireless communication technique in such an environment relates to the avoidance of much of the costs associated with the cabling of the network. However, most of these WLAN products failed to meet several key criteria essential to wide-scale adoption. The system failed to be fast (e.g. at least 10 Megabits per second), was not simple (e.g. plug and play) and was not economical (e.g. less than $500 per node).
In a manner analogous to the growth of the wired LANs, initial application and market success of the WLAN was in specialized, vertical markets. Thus, applications that highly valued the mobile, untethered connectivity were the early targets of the WLAN industry. Over the last two years, however. WLANs began to emerge as an option to fill an ever-widening gap in the corporate enterprise infrastructure. Currently, there are three basic types of wireless networks: wide-area network (“WAN”), local area networks (“LAN”) and campus area networks (“CAN”).
These WLANs, however, exhibit one drawback or another such as being too slow, restricted to certain environments or too expensive. In an ideal environment, the optimal WLAN solution would eliminate these shortcomings and exhibit the characteristics of being easy to install, having performance of over 10 Megabits per second (“Mbps”), being cost effective and being compatible with existing Ethernet networking equipment. Unfortunately conventional WLANs do not meet more than a few of these requirements with some failing to satisfy even one.
Today's fourth generation (“4G”) of WLAN technology attempts to resolve these problems as well as increase bandwidth and avoid the interference associated with the more crowded lower frequency bands by operating within the 5 GHz frequency band. This increase in frequency enables such 4G WLANs to operate at data rates of approximately 10 Mbps. Such WLANs typically operate in the 5.775-5.850 GHz Industrial, Scientific, and Medical (“ISM”) band as well as within frequency bands at 5.2 GHz and 5.3 GHz, Unlicensed-National Information Infrastructure (U-NII) bands. These three segments have been designated by the FCC as exclusively for high speed data transmission. Unlike the lower frequency bands used in prior generations of WLANs, the 5 GHz bands do not have a large number of potential interferors, such as microwave ovens or industrial heating systems. In addition, there is much more bandwidth available at 5 GHz-350 MHz as compared to the 83 MHz within the 2.4 GHz band and the 26 MHz within the 900 MHz band. This combination of greater available bandwidth and reduced sources of interference makes the 5 GHz bands a desirable frequency range in which 4G WLANs have performance comparable to that achieved by wired networks.
Unfortunately, multipath interference, or Rayleigh fading, occurs when radio waves reflect off the surface of physical objects, producing a complex pattern of interfering waves which causes signals to meet at the antenna and cancel each other out. In addition, further interference is generated by other devices, which operate in the 2.4 GHz frequency range. These forms of interference present a design challenge for conventional WLAN systems, which have resulted in many conventional 3G WLANs relying upon a spread spectrum scheme. Unfortunately, by overcoming this interference with spread spectrum technology, such as Frequency Hopping Spread Spectrum (“FHSS”) and Direct Sequence Spread Spectrum (“DSSS”), these 4G WLANs are limited to data rates of up to 1 Mbps to 2 Mbps.
While FHSS and DSSS. at first, may appear to be simpler to implement than other schemes, there still are some very subtle difficulties that occur when strong interfering signals exist in such a system. For example, the basis of the noise immunity within a DSSS-based system is the fact that the desired signal and interference (or noise) are uncorrelated. In complex interference environments, which are becoming more common as usage increases, particularly ones in which very strong signals may be present, non-linearities in the receiver generate intermodulation (“IM”) distortion products between the desired signal and the interfering signals. These IM products now are correlated with the desired signal, thus reducing the resulting signal to noise ratio when processed in the receiver. Therefore, even though the use of spread spectrum techniques combined with more available bandwidth and more complex modulation schemes allows such WLANs to operate at higher data rates, those data rates are not as high as desired (e.g. over 10 Mbps).
Therefore, there currently is a need for a WLAN system, which can achieve higher data rates while still maintaining relative immunity to interference, such as multipath propagation interference.